Step motor

ABSTRACT

A step motor includes a rotor having four magnetic poles, a first magnetic pole magnetically excited by a first coil, a second magnetic pole magnetically excited by a second coil, and a third magnetic pole magnetically excited by the first coil and the second coil. A gap between the third magnetic pole and the rotor is smaller than that between the first magnetic pole and the rotor and that between the second magnetic pole and the rotor. In the step motor, it is possible to intensify the influence, on the rotor having four poles, of the magnetic fields created from three magnetic poles. Accordingly, this can weaken the magnetic fields set up from the first magnetic pole and the second magnetic pole, which tend to intensify the influence on the rotor, enabling the torque smaller at the time of the rotation start of the rotor.

BACKGROUND OF THE INVENTION

1. Field of the Invention

This invention generally relates to small-sized step motors for use incameras or the like, and more particularly, to a step motor suitable foruse in driving a shutter, lens, barrier, and the like of a camera.

2. Description of the Related Art

In recent years, cameras have become electronics devices and shuttersthereof are driven by the step motors. In addition, cameras are promotedto be small-sized and light-weighted, leading to a demand forsmall-sized step motors with high accuracy. For example, JapaneseExamined Patent Publication No. 2-2382 discloses a step motor thatsimultaneously excites two coils provided on a stating element (stator)and rotates a rotating element (rotor).

FIG. 7 is a plan view schematically showing a step motor 100 disclosedin Japanese Examined Patent Publication No. 2-2382. A stator 103 havinga shape of isosceles trapezoid is provided in an outer periphery of arotor 101. The stator 103 includes three magnetic poles 104, 105, and106, and also includes a first coil 108 on the left and a second coil109 on the right. The rotor 101 composed of a permanent magnet isrotated by controlling the direction of the current applied to the coils108 and 109 to change the direction of the magnetic field. In addition,the third magnetic pole 106 is magnetically excited by left and rightcoils, and in particular, the third magnetic pole 106 includes aprotrusion portion 107 that comes close to the rotor 101. The protrusionportion 107 is provided so that a North magnetic pole (or South magneticpole) of the rotor 101 is positioned to the protrusion portion 107 whenno magnetic field is set up.

It is preferable, however, that the step motor be designed to start therotation from a halting state with less power consumption. However, therotor 101 of the above-mentioned step motor 100 has two magnetic poles,and it is configured that South magnetic pole or North magnetic pole ispositioned to the protrusion portion 107 in a halting state. Therefore,the above-described step motor 100 has a drawback that a large amount ofenergy is consumed at the time of starting the rotation.

SUMMARY OF THE INVENTION

It is an object of the present invention to solve the above-mentioneddrawback and provide a step motor in which the power consumption can besuppressed at the time of starting rotation of the rotor.

According to another aspect of the present invention, there is provideda step motor including a rotor having four magnetic poles; a firstmagnetic pole magnetically excited by a first coil; a second magneticpole magnetically excited by a second coil; and a third magnetic polemagnetically excited by the first coil and the second coil. A gapbetween the third magnetic pole and the rotor is smaller than thatbetween the first magnetic pole and the rotor and that between thesecond magnetic pole and the rotor. According to the present invention,the gap between the third magnetic pole and the rotor is smaller thanthat between the first magnetic pole and the rotor and that between thesecond magnetic pole and the rotor. By employing such configuration, theeffect of the magnetic field can be intensified from the third magneticpole to the rotor having four magnetic poles. Accordingly, it ispossible to relatively weaken the magnetic fields set up from the firstmagnetic pole and the second magnetic pole, which tend to intensify theinfluence on the rotor, enabling the power consumption to be reduced atthe time of starting the rotation of the rotor. Thus, it is possible toprovide the step motor, which can save energy and consume less power andgenerate a high drive torque at the time of starting the rotation.

BRIEF DESCRIPTION OF THE DRAWINGS

Preferred embodiments of the present invention will be described indetail with reference to the following drawings, wherein:

FIG. 1 is a view showing main components of a step motor in accordancewith an embodiment;

FIGS. 2A through 2E are views showing a case where a rotor of the stepmotor is rotated in a two-phase magnetically excited state in accordancewith an embodiment;

FIGS. 3A through 3E are views showing a case where a rotor of the stepmotor is rotated in a clockwise direction in a one-phase magneticallyexcited state in accordance with an embodiment;

FIGS. 4A through 4E are views showing a case where a rotor of the stepmotor is rotated in a counterclockwise direction in a one-phasemagnetically excited state in accordance with an embodiment;

FIG. 5 is a view showing a stator having a preferred shape suitable foruse in the step motor;

FIG. 6 is a perspective view showing an appearance of a module having astructure of the step motor; and

FIG. 7 schematically shows a conventional step motor.

DESCRIPTION OF THE PREFERRED EMBODIMENTS

A description will now be given, with reference to the accompanyingdrawings, of an embodiment of the present invention. FIG. 1 is a viewshowing main components of a step motor in accordance with an embodimentof the present invention. A step motor 1 includes a rotor 2 and a stator3, the rotor 2 being arranged in the center thereof and capable ofrotating in both directions, the stator 3 being arranged to face anouter circumference of the rotor 2. The rotor 2 has a cross-section ofcircle and has a shape of cylinder. The stator 3 is integrally formed tohave a plan view of substantially lateral U-shape, and is located in astate that the rotor 2 is housed in an internal space thereof. Inaddition, the step motor 1 is shown in FIG. 1 with an open end of thelateral U-shape facing upwardly.

The rotor 2 includes four magnetic poles, which are composed of twoNorth magnetic poles and two South magnetic poles. The rotor 2 is apermanent magnet magnetized in positions where the poles having a samepolarity face each other, and is rotatably provided in both directionsaround an axis 21. Both ends of the stator 3 having the above-mentionedlateral U-shape are formed to face a circumferential surface of therotor 2. The both ends are respectively a first magnetic pole 11 and asecond magnetic pole 12. Also, a third magnetic pole 13 is arranged inan intermediate position of the first magnetic pole 11 and the secondmagnetic pole 12.

A first coil 4 is wound between the first magnetic pole 11 and the thirdmagnetic pole 13, and a second coil 5 is wound between the secondmagnetic pole 12 and the third magnetic pole 13. The first magnetic pole11 is magnetically excited when a current flows through the first coil4, and the second magnetic pole 12 is magnetically excited when acurrent flows through the second coil 5. In contrast, the third magneticpole 13 is magnetically excited by both the first coil 4 and the secondcoil 5. Accordingly, a magnetically excited state of the third magneticpole 13 is shown as a combination of current-flowing states of the firstcoil 4 and the second coil 5.

FIG. 1 shows a current control circuit 25, which is indicated as adotted line and connected to the first coil 4 and the second coil 5 ofthe step motor 1. In accordance with the present embodiment, the currentcontrol circuit 25 supplies the current to magnetically excite the firstcoil 4 and the second coil 5. Such supplied current includes twopatterns that have been set. In a first pattern, the current is suppliedfrom the current control circuit 25 to magnetically excite both thefirst coil 4 and the second coil 5, and a drive state of the rotor 2 iscontrolled by changing a current supply direction in each coil. In thefirst pattern, there are two state: one state is that the first magneticpole 11 and the second magnetic pole 12 are magnetically excited to havea same magnetic polarity and the other state is that the first magneticpole 11 and the second magnetic pole 12 are magnetically excited torespectively have different types of magnetic polarity. At this time, ifthe first magnetic pole 11 and the second magnetic pole 12 aremagnetically excited to have the same magnetic polarity, which resultsin that the third magnetic pole 13 sets up a stronger magnetic fieldthan those of the afore-mentioned magnetic poles. On the contrary, ifthe first magnetic pole 11 and the second magnetic pole 12 aremagnetically excited to respectively have different types of magneticpolarity, magnetization in the third magnetic pole 13 is cancelled eachother, resulting in a non-magnetization state.

In a second pattern, the current is supplied from the current controlcircuit 25 to magnetically excite either the first coil 4 or the secondcoil 5, and the drive state of the rotor 2 is controlled by changing thecurrent supply direction. In the second pattern, only either the firstmagnetic pole 11 or the second magnetic pole 12 is magnetically excited,and it is changed to have an opposite magnetic polarity by changing thecurrent supply direction. In the second pattern, the third magnetic pole13 is magnetically excited to have an opposite polarity of those of thefirst magnetic pole 11 and the second magnetic pole 12 that has beenmagnetically excited.

In the first pattern, the drive of the rotor 2 is controlled by atwo-phase magnetically excited state in which the first coil 4 and thesecond coil 5 are magnetically excited. On the other hand, in the secondpattern, the drive of the rotor 2 is controlled by a one-phasemagnetically excited state in which only either of the first coil 4 orthe second coil 5 is magnetically excited. A description will be givenlater in detail, with reference to the drawings, of rotation states ofthe rotor 2 in the first pattern and in the second pattern.

In the step motor 1, the rotor 2 is configured to include four magneticpoles, and in particular, is configured to be capable of reducing thetorque. This point will now be described. In the step motor 1, a samegap d is configured to be a distance between the circumferential surfaceof the rotor 2 and the first magnetic pole 11 and a distance between thecircumferential surface of the rotor 2 and the second magnetic pole 12.In contrast, a gap D is a distance between the circumferential surfaceof the rotor 2 and the third magnetic pole 13, and is configured to besmaller than the gap d. The gap D is configured to be narrow so as tointensify the magnetic field set up from the third magnetic pole 13 towork on the rotor 2 and relatively weaken the magnetic field set up fromthe first magnetic pole 11 and that set up from the second magnetic pole12 to work on the rotor 2. Preferably, the gap D is configured to beapproximately 0.3 to 1 times as long as the gap d. More preferably,approximately 0.8 times.

If the magnetic fields set up from the first magnetic pole 11 and thesecond magnetic pole 12 intensely work on the rotor 2, this increasesdetent torque of the rotor 2 in the halting state. Accordingly, in thestep motor 1, the gap D between the circumferential surface of the rotor2 and the third magnetic pole 13 is configured to be small and intensifythe influence of the third magnetic pole 13, so as to relativelydecrease the magnetic influence produced from the first magnetic pole 11and the second magnetic pole 12 that work on the rotor 2. Thus, thedetent torque is decreased by averaging the magnetic balance between thestator 3 and the rotor 2. This makes it possible to reduce the powerconsumption in the step motor 1. The conventional step motor asdescribed above is also configured in such a manner that the thirdmagnetic pole is arranged close to the rotor. The rotor in theaforementioned step motor includes two magnetic poles, and a thirdmagnetic pole is arranged close to the rotor so that one of the magneticpoles in the rotor stops to face the third magnetic pole. However, inthe step motor 1 in accordance with an embodiment, the third magneticpole 13 is arranged close to the surface of the rotor 2 so that themagnetic fields set up from the respective magnetic poles to work on therotor 2 are balanced to reduce the torque. In this manner, differenteffects are obtainable with seemingly similar configurations. This isbecause the step motor 1 in accordance with an embodiment of the presentinvention employs the rotor 2 having four magnetic poles.

Hereinafter, a description will be given of, with reference to FIG. 2Athrough FIG. 4E, the rotation of the rotor 2 in the step motor 1. FIGS.2A through 2E show the above-described first current supply pattern, andthe rotor 2 is rotated by the two-phase magnetically excited state thatexcites the first coil 4 and the second coil 5. FIG. 3A through FIG. 4Eshow the above-described second pattern, and the rotor 2 is rotated by aone-phase magnetically excited state that excites only either the firstcoil 4 or the second coil 5. In particular, FIGS. 3A through 3E show acase where the first coil 4 is magnetically excited, and FIGS. 4Athrough 4E show a case where the first coil 4 is magnetically excited.The current is supplied to the coils 4 and 5 shown in FIG. 2A throughFIG. 4E by the current control circuit 25 shown in FIG. 1, yet it is notshown in these drawings. In addition, FIG. 3A through FIG. 4E show onlycoils through which the current is flown to facilitate theunderstanding.

Referring to FIGS. 2A through 2E, a description will be given of how therotor 2 of the step motor 1 rotates. FIGS. 2A through 2E show theabove-mentioned first pattern, and also show a case where the first coil4 and the second coil 5 are magnetically excited to rotate the rotor 2in a clockwise direction (in a right-hand direction) at a step angle of45°. In FIG. 2A, there is no current flowing through the coils 4 and 5.In FIG. 2B through FIG. 2E, the current is controlled to supply to thecoils 4 and 5 and rotate the rotor 2 in a clockwise direction in atime-series manner. In FIG. 2A, the first magnetic pole 11 and thesecond magnetic pole 12 are not magnetically excited, and South magneticpole and North magnetic pole of the rotor 2 are respectively arranged toface the first magnetic pole 11 and the second magnetic pole 12.

In FIG. 2B, the current flows through the first coil 4 and the secondcoil 5 from the state shown in FIG. 2A, and both the first magnetic pole11 and the second magnetic pole 12 are magnetically excited to be Southmagnetic pole. At this time, the magnitude of North magnetic pole isdoubled and excited in the third magnetic pole 13. Next, in FIG. 2C, theexcitation state in the first magnetic pole 11 is retained in Southmagnetic pole from the state shown in FIG. 2B, and the second magneticpole 12 is magnetically excited to be reversed to North magnetic pole.At this time, North magnetic pole and South magnetic pole aremagnetically excited and cancelled each other, resulting in nomagnetization in the third magnetic pole 13. In the same manner, in FIG.2D, the first magnetic pole 11 and the second magnetic pole 12 are bothmagnetically excited to be North magnetic pole from the state shown inFIG. 2C. At this time, the magnitude of North magnetic pole is doubledand excited in the third magnetic pole 13. Next, in FIG. 2E, theexcitation state in the first magnetic pole 11 is retained in Northmagnetic pole from the state shown in FIG. 2D, and the second magneticpole 12 is magnetically excited to be opposite, namely, to be Southmagnetic pole. At this time, North magnetic pole and South magnetic poleare magnetically excited and cancelled each other, resulting in nomagnetization in the third magnetic pole 13.

As described, the rotor 2 is rotated in a clockwise direction in stepsof 45° as shown in the drawings, as the magnetization state graduallychanges in the magnetic poles 11 through 13 in the stator 3. Here, therespective drawings of FIGS. 2A through 2E show that the current flowsthrough the first coil 4 and the second coil 5 and the rotor 2 islocated in a position where a rotation of 45° is completed. As shown inFIG. 2B, when the coils 4 and 5 are magnetically excited by the currentsupplied from the current control circuit 25 and changed from thehalting state of the rotor 2 shown in FIG. 2A, the torque is applied tothe rotor 2 in a clockwise direction. When the rotor 2 starts rotatingin a clockwise direction, the step motor 1 is capable of starting therotation with relatively less power consumption, because the thirdmagnetic pole 13 is arranged close to the surface of the rotor 2. Thatis to say, a current amount supplied from the current control circuitcan be reduced, enabling the rotor 2 to be rotated for purposes ofsaving energy.

FIGS. 3A through 3E show the above-mentioned second current supplypattern, and also show a case where only the first coil 4 ismagnetically excited in a one-phase magnetically excited state to rotatethe rotor 2 in a clockwise direction (in a right-hand direction) at astep angle of 90°. In FIG. 3A, there are no currents flowing across thecoils 4 and 5. In FIG. 3B through FIG. 3E, the current is controlled tosupply to the coil 4 to rotate the rotor 2 in a clockwise direction insteps of 90° in a time-series manner. In FIGS. 3A through 3E, a polarityof the first magnetic pole 11 is reversed by reversing the currentdirection supplied to the first coil 4. At this time, the polarity ofthe third magnetic pole 13 is an opposite one of the first magneticpole. In addition, the second magnetic pole 12 is not magneticallyexcited from the coil, and has the same polarity as that of the thirdmagnetic pole 13.

First, in FIG. 3A, the first magnetic pole 11 and the second magneticpole 12 are not magnetically excited, and South magnetic pole and Northmagnetic pole of the rotor 2 are respectively arranged to face the firstmagnetic pole 11 and the second magnetic pole 12, same as shown in FIG.2A. In FIG. 3B, next, the current flows across the first coil 4 from thestate of FIG. 3A, and the first magnetic pole 11 is magnetically excitedto be South magnetic pole. At this time, the third magnetic pole 13 andthe second magnetic pole 12 are magnetically excited to be Northmagnetic pole. In FIG. 3C, which is subsequently shown, the firstmagnetic pole 11 is changed to be North magnetic pole in themagnetically excited state from the state shown in FIG. 3B, and thethird magnetic pole 13 and the second magnetic pole 12 are magneticallyexcited to be reversed to South magnetic poles. In the same manner, inFIG. 3D, the first magnetic pole 11 is magnetically excited to be Southmagnetic pole from the state shown in FIG. 3C. At this time, the thirdmagnetic pole 13 and the second magnetic pole 12 are magneticallyexcited to be North magnetic poles. In FIG. 3E, next, the first magneticpole 11 is changed to be North magnetic pole from the state shown inFIG. 3D, and the third magnetic pole 13 and the second magnetic pole 12are magnetically excited to be opposite, namely, to be South magneticpole.

As described above, the rotor 2 is rotated in a clockwise direction insteps of 90° as shown, with the magnetization state gradually changingin the magnetic poles 11 through 13 in the stator 3. Here, therespective drawings of FIGS. 3A through 3E show that the current flowsthrough the first coil 4 and the rotor 2 is located in a position wherethe rotation of 90° is completed. As shown in FIGS. 3A through 3E, whenthe coil 4 is magnetically excited by the current supplied from thecurrent control circuit 25 and the rotor 2 starts rotating in aclockwise direction as shown in FIG. 3B from the halting state of therotor 2 shown in FIG. 3A, the step motor 1 is capable of starting therotation with relatively less power consumption, because the thirdmagnetic pole 13 is arranged close to the surface of the rotor 2. Thisenables the rotor 2 to be rotated for purposes of saving energy. Here,the example shown in FIGS. 3A through 3E exhibits a remarkable advantagethat the rotor 2 can be rotated in steps of 90° in a clockwise directionwith the second coil 5 being in a halting state.

Further, FIGS. 4A through 4E show the above-mentioned second currentsupply pattern, and also show a case where only the second coil 5 ismagnetically excited in a one-phase magnetically excited state to rotatethe rotor 2 in a counterclockwise direction at a step angle of 90°.FIGS. 4A through 4E show accurately reverse operations of those in FIGS.3A through 3E. That is to say, in FIG. 4A, there is no current flowingthrough the coils 4 and 5. In FIG. 4B through FIG. 4E, the current iscontrolled to supply to the coil 5 to rotate the rotor 2 in acounterclockwise direction in a time-series manner. In FIGS. 4A through4E, the polarity of the second magnetic pole 12 is reversed by reversingthe current supplied to the second coil 5. Then, the polarity of thethird magnetic pole 13 is opposite of that of the second magnetic pole12. In addition, the first magnetic pole 11 is not magnetically excitedfrom the coil, and has the same polarity as that of the third magneticpole 13.

The rotor 2 rotates in a counterclockwise direction in steps of 90° asalso shown in FIGS. 4A through 4E, with the magnetization stategradually changing in the magnetic poles 11 through 13 in the stator 3.Also, the third magnetic pole 13 is arranged close to the surface of therotor 2 when the rotor 2 starts rotating in a counterclockwisedirection. This makes it possible to start the rotation at a relativelysmall torque and suppress the power consumption. FIGS. 4A through 4Eshow a case where the rotor 2 can be rotated in a counterclockwisedirection in steps of 90° with the first coil 4 being in a haltingstate.

FIG. 5 is a view showing a stator having a preferable shape suitable foruse in the step motor 1. In FIG. 5, the same components andconfigurations as those of FIG. 1 and FIGS. 2A through 2E have the samereference numerals. The first magnetic pole 11 and the second magneticpole 12 of the stator 3 are configured to face the circumferentialsurface of the rotor, not shown, and to be formed into a shape having avertically longer side to correspond to the longer side of the rotor.The stator 3 includes arms 31 and 32 on both sides, and the arms 31 and32 are connected to a base 35. The third magnetic pole 13 is formed inthe center of the base 35. The third magnetic pole 13 is also formed tohave a shape having a vertically longer side, same as those of the firstmagnetic pole 11 and the second magnetic pole 12. The magnetic poles 11through 13 are formed to have such a vertically longer side and extendin both directions of thickness of the stator 3. In addition, the shapehaving a vertically longer side may be formed by extending in eitherdirection of thickness, although the length becomes the half.

The stator 3 includes the arms 31 and 32, around which the coils 4 and 5are wound for magnetically exciting the first through third magneticpoles. To position the coils 4 and 5, projections 33 and 34 are providedon back ends of the arms. Such provided projections 33 and 34 arecapable of realizing the structure, in which the coils 4 and 5respectively wound around the arms 31 and 32 can be surely positioned.In addition, indentations 37 through 39 are formed on tops of themagnetic poles 11 through 13. The step motor 1 in accordance with thepresent embodiment is incorporated into a module together with upper andlower cases. The indentations 37 through 39 are for use in positioningthe case to be set.

FIG. 6 is a perspective view showing an appearance of the step motor 1,which is incorporated into a module with main components thereof. Alsoin FIG. 6, the same components and configurations as those of FIG. 1 andFIGS. 2A through 2E have the same reference numerals. FIG. 6 shows amodule in which an upper case 7 and a lower case 8 are set together withthe main components from top and bottom. Such step motor module isemployed for a shutter driving portion of a camera, enabling the stepmotor that consumes less power at the time of activation. It is possibleto set the step angle at 45°, 90°, 135°, 180°, and more, by controllingthe current with the current control circuit 25. This makes it possibleto provide the step motor with high flexibility, in which the step angleis adjustable.

The afore-mentioned rotor may have a shape of cylinder; a stator havinga plan view of substantially lateral U-shape is arranged to face acircumferential surface of the rotor; the first magnetic pole and thesecond magnetic pole are provided on both ends of the stator; and thethird magnetic pole is provided in the center of the stator. Inaddition, preferably, in the afore-mentioned step motor, the first coilis provided between the first magnetic pole and the third magnetic poleand the second coil is provided between the second magnetic pole and thethird magnetic pole; and the stator includes protrusions for preventingdisplacement of the first coil and the second coil. With suchprotrusions, the positions of the first coil and the second coil arestable, and it is possible to provide the step motor having a morepreferable structure.

Additionally, the first coil and the second coil are connected to acurrent control circuit that magnetically excites them. It is possibleto control the rotation of the stator in a two-phase magneticallyexcited state by changing the current directions to be applied from thecurrent control circuit to the first coil and the second coil.Furthermore, the first coil and the second coil are connected to acurrent control circuit that magnetically excites them. The rotation ofthe stator may be controlled in a one-phase magnetically excited stateby changing the current direction to be applied from the current controlcircuit to either the first coil or the second coil. When the current issupplied to magnetically excite the first coil and the second coil so asto control the drive of the rotor, the rotor may be rotated at a rotorstep angle of 45°. Also, when the current is supplied to magneticallyexcite either the first coil or the second coil so as to control thedrive of the rotor, the rotor may be rotated at a rotor step angle of90°. Accordingly, it is possible to provide the step motor with highflexibility, because the step angle can be changed as described.

ADVANTAGEOUS EFFECT OF THE INVENTION

According to the present invention, it is possible to intensify theinfluence, on a rotor having four magnetic poles, of the magnetic fieldset up from a third magnetic pole provided on a stator. This makes itpossible to relatively weaken the magnetic fields set up from the firstmagnetic pole and the second magnetic pole, which intend to intensifythe influence on the rotor, and provide a step motor by which a rotordetent torque is small and power consumption is suppressed.

The present invention is not limited to the above-mentioned embodiments,and other embodiments, variations and modifications may be made withoutdeparting from the scope of the present invention.

1. A step motor comprising: a rotor having four magnetic poles; a firstmagnetic pole magnetically excited by a first coil; a second magneticpole magnetically excited by a second coil; and a third magnetic polemagnetically excited by the first coil and the second coil; wherein agap between the third magnetic pole and the rotor is smaller than thatbetween the first magnetic pole and the rotor and that between thesecond magnetic pole and the rotor.
 2. The step motor as claimed inclaim 1, wherein: the rotor has a shape of cylinder; a stator having aplan view of substantially lateral U-shape is arranged to face acircumferential surface of the rotor; the first magnetic pole and thesecond magnetic pole are provided on both ends of the stator; and thethird magnetic pole is provided in the center of the stator.
 3. The stepmotor as claimed in claim 2, wherein: the first coil is provided betweenthe first magnetic pole and the third magnetic pole and the second coilis provided between the second magnetic pole and the third magneticpole; and the stator includes protrusions for preventing displacement ofthe first coil and the second coil.